1. Field of the Invention
The present invention relates generally to a a method for making a phosphor layer of a cathode ray tube and particularly concerns for making the phosphor layer through sedimentation method utilizing centrifugal force.
2. Description of the Prior Art
In a small cathode ray tube having a face panel of three inch diameter or less, it is desirable to have a phosphor layer of an especially fine and dense configuration and a uniform thickness.
In general, a phosphor layer of a monochrome cathode ray tube comprises phosphor powder having medium grain size (d.sub.50) of about 7 .mu.m and is made by the sedimentation method. In forming of the phosphor layer, the phosphor powder is dispersed in an aqueous solution containing water glass and an electrolyte for accelerating settling (cushion solution), and the solution is put in the glass bulb of a cathode ray tube. Then, after the gelled water glass and the phosphor powder make natural sedimentation, supernatant liquid is removed and the film on the bottom of the flask-shaped glass bulb is dried. The phosphor layer made in the above-mentioned conventional settling method does not have a satisfactory fine configuration nor uniformity of thickness as a small type cathode ray tube. On the other hand, it is well known that when small phosphor grain size is selected in order to obtain a phosphor layer of very fine configuration, not only does the sedimentation take a long time, but also undesirable coagulation induces a lack of smoothness, hence producing a phosphor layer of uneven surface.
As a measure to obtain an even phosphor layer, a sedimentation method utilizing centrifugal force has been disclosed in the Japanese Examined Published Patent Application No. Sho 50-745. In the conventional sedimentation method using the centrifugal force, however, a phosphor layer is to be formed on a glass sheet supported in a revolving enclosure, apart from the bottom, and therefore the utilization rate of the phosphor slurry is very low. And furthermore, it has been difficult to produce a phosphor layer having a uniform thickness on the whole area of the glass sheet. Still furthermore, in the above-mentioned conventional method, unless the phosphor layer sticking in the revolving enclosure is removed every time after one sedimentation process has been completed, producing of the phosphor layer in the next sedimentation process becomes impossible.